The present invention relates to a method for generating a spreading code used in a CDMA (Code Division Multiple Access) and a method for selecting a spreading code having the small time dependence from the generated codes, more preferably a method for generating and selecting orthogonal codes of different spreading factors.
In a CDMA system, information signals are spread using inherent spreading codes assigned for all users, which identify each one of them. That enables signals of a plurality of users to be multiplied in the same frequency band, which results in a large efficient use of a frequency. In a CDMA system, if spreading codes absolutely orthogonalize, no interference occurs between signals of each user. That allows the number of users corresponding to the number of orthogonal codes to be accommodated under the ideal propagation condition, which ensures no quality deterioration in communications
And, in a CDMA system, in the case of multiplying spreading signals using spreading codes having different spreading factors, it is possible to multiply user signals having different rates in the same frequency band because the division of signals is possible when different spreading codes are used. Especially, using Walsh codes as a spreading code makes such communication possible because Walsh codes having different spreading factors are orthogonalized to each other.
FIG.1 illustrates a case of transmitting information signals from user #1 to user #7 using spreading codes having different spreading factors. User #1, #2 and #3 use a spreading code of spreading factor 8, user #4 and user #5 use a spreading code of spreading factor 16, and user #6 and user #7 use spreading code of factor 32. Each number from 1 to 7 in FIG. 1 respectively illustrates transmission signals of each of user #1 to user #7.
As illustrated in FIG. 1, when a spreading factor of a spreading code becomes a second, i.e. spreading factor 16 becomes 8, the time for one symbol becomes shorter. That makes a symbol rate twice, which results in twice of information capacity. However with respect to other users, it is impossible to transmit signals using a spreading code of spreading factor 8 and its two-combined spreading code of spreading factor 16. Because of it, the total information capacity that can be transmitted is not improved in the whole by making spreading factor of a spreading code one second.
FIG.2 illustrates examples of Walsh code. In this example, a length of spreading code is 16, which means the presence of 16 codes orthogonalized to each other. These codes are referred to W1 to W16.
For instance, when 0000 is used as a spreading code of spreading factor 4, this spreading code does not orthogonalizes with W4, W8, W12 and W16. It means that a user in using this spreading factor 4 consumes four codes of spreading factor 16 for four users. On the other hand, the transmission information capacity becomes 4 times that of the case of using a spreading code of spreading factor 16, which provides no change to the total amount of transmission information in the system. Namely, the total transmission information capacity (spreading factorxc3x97transmission information capacity) is constant when Walsh codes are used even by combining any spreading factors.
However, since the time dependence of Walsh code is large, its performance deteriorates drastically by delayed waves. To prevent it, a method to use Walsh codes with long codes an others are tried, however in this case a problem in acquiring synchronization is still remained.
On the other hand, the time dependence of orthogonal Gold code well known as an orthogonal code is smaller than that of Walsh code. Accordingly, the deterioration rate in its performance caused by delayed waves is smaller than that of Walsh code. However, in orthogonal Gold codes, while those of the same spreading factor orthogonalize, those of different spreading factors necessarily orthogonalize. Because of it, interference between user signals occurs, which results in deterioration in quality and decreases of transmission capacity.
And, in the case of using codes having large time dependence, interference between codes caused by delayed waves becomes large, which may bring deterioration in quality. The more the transmission power is increased to compensate it, the more interference to other users is increased. That results in deterioration in quality and increases of transmission capacity in the whole system.
The present invention is carried out taking into account the problems described above and has an object to provide a method for generating spreading codes orthgonalizing, even in the case of different spreading factor, and having low time dependence, and a method for selecting spreading codes having smaller time dependence from the generated spreading codes.
The object is achieved by a code generating method for combining an orthogonal code of a specific spreading factor for CDMA and its polarity inverted orthogonal code to generate an orthogonal code having a larger spreading factor than that of the orthogonal code.
And, the object is achieved by a code selecting method comprising a step for combining an orthogonal code of a specific spreading factor for CDMA and its polarity inverted orthogonal code to generate an orthogonal code having a larger spreading factor than that of the orthogonal code and a step for assigning the generated orthogonal code for a user in using an orthogonal code having a smaller spreading factor.